Each of Japanese Patent Publication No. 3,733,085 and Japanese Patent Publication No. 3,826,124 discloses a plastic bag formed of two sheets of panel material and a pair of sheets of side gusset material. The sheets of panel material are superposed with each other to have opposite side edges. The sheets of side gusset material extend along the opposite side edges. In addition each of the sheets of side gusset material is folded into halves and interposed between the sheets of panel material to have a folded inner edge and open outer edges.
Each of the sheets of side gusset material further has opposite end portions one of which is folded obliquely along a folded line as it is folded into halves so that a triangular flap is formed by the end portion to be shaped into a triangle having a base formed by the folded line. Each of the sheets of side gusset material further has opposite end edges. The triangle further has an apex formed by an intersection between one of the opposite end edges and the folded inner edge of each of the sheets of side gusset material. The triangular flap has a folded hypotenuse formed by the folded inner edge. The triangular flap further has an open hypotenuse formed by the end edge.
The sheets of panel material, the sheets of side gusset material and the triangular flaps are heat sealed with each other along the opposite side edges of the sheets of panel material. The sheets of panel material and the sheet of side gusset material can therefore be closed to each other by means of the triangular flap with a sealing strength enough to be free of leakage from the end portion of the sheet of side gusset material.
Japanese Patent Publication No. 5, 631,154 also discloses the plastic bag including the triangular flap formed of each of the sheets of side gusset material. In addition, in the plastic bag, the triangular flap includes a flat surface in which an aperture is formed to discharge air through the aperture. The aperture is intended to discharge gas through the aperture in addition to the air, when the gas generates in the plastic bag. In this case, insects or foreign substances are less likely to enter the aperture, in particular when the aperture is small.
However, in order to make the plastic bags successively in an apparatus, in which each of the plastic bags includes the aperture formed in the flat surface of the triangular flap, the apparatus has to be arranged to make the sheet of side gusset material punched by a punch blade before being supplied so that the aperture can be formed in the sheet of side gusset material. As a result, the apparatus must be complicated in structure and high in cost. It is therefore desired to discharge the air or the gas in a way different from the plastic bag of the publication.
It is therefore an object of the invention to provide an improved plastic bag including the triangular flap formed of each of the sheets of side gusset material, to discharge air or gas in a way different from the prior art.